1. Field of the Invention
This invention relates to a unit for measuring optical properties of optical elements. The optical properties such as transmittance, reflectance, phase change in transmission or reflection are measured.
2. Background of the Related Art
It is well known that typical optical properties of optical elements include transmittance and reflectance. The transmittance and the reflectance respectively include relative transmittance and relative reflectance that are defined as a relative value of an object to a predetermined value of a reference object. The transmittance and the reflectance also include respectively absolute transmittance and absolute reflectance that are determined by measuring the object itself. The transmittance change and the reflectance that change with respect to wavelength are called spectral transmission factor and spectral reflection factor, respectively.
In order to measure the transmittance or the reflectance, generally, the spectral transmission factor or the spectral reflection factor is measured, and also, the transmittance or the reflectance with respect to angle of incidence is considerably often measured.
Another optical properties of the optical elements include refractive index of materials of the optical elements, and optical constant of thin film. The refractive index and the optical constant are generally obtained by measuring phase change dependence on angle of incidence, which is caused when light passes through an object, and by analyzing the measured phase change (hereafter this phase change is called "phase change in transmission"). The refractive index and the optical constant are also obtained by measuring phase change dependence on angle of incidence, which is caused when light is reflected by the object, and by analyzing the measured phase change (hereafter this phase change is called "phase change in reflection").
The transmittance and the reflectance are commonly measured by a commercial spectrophotometer along with an attached special optical system. The phase change in transmission and reflection is commonly measured by an ellipsometer. The following explain these conventional apparatus.
The conventional apparatus measure the transmittance of the optical elements using the spectrophotometer, considering the dependence on the angle of incidence, as described below. FIG. 25 shows a conventional apparatus, but the details of the spectrophotometer, i.e., a monochromator portion and a light receptor, are not shown.
In this conventional apparatus, an object 11 is set on a goniometer 13, and the object 11 is rotated predetermined degrees on the goniometer 13. The amount of light that has past through the object 11 (the amount of light to be measured) is measured, changing an angle of the incident light .theta.. The reference amount of light is measured in a state that the object 11 is not set on the goniometer 13. From the measured amount of light and reference amount of light, the transmittance dependence on an angle of incidence can be measured.
A conventional apparatus using so-called VN method that is written in Japanese Industrial Standard JISK 0115-1992 is known as an apparatus for measuring the absolute reflectance of an optical element. FIGS. 26(A) and 26(B) explain this apparatus.
The VN method uses three mirrors, M1, M2 and M3. These mirrors M1 through M3 are arranged so that an optical path extending from the mirrors M1 through M3 is formed like a "V" letter (FIG. 26(A)), and this stat is called the first state. The amount of light output from the optical system (the reference amount of light) is measured by the light receptor (not shown). The reference amount of light lr is EQU lr=lo.times.R1.times.R2.times.R3
where lo is the amount of incident light, R1 through R3 are reflectances of mirror M1 through M3, respectively.
Then, the object 11 is inserted onto the optical path of the optical system, as shown in FIG. 26(B). The mirrors M1 through M3 are moved and rotated so that the object 11 and Mirrors M1 through M3 produce an optical path like an "N" letter. The amount of light output from the optical system (the amount of light to be measured) is measured by the light receptor. The amount of light is EQU ls=R1.times.R2.times.R3.times.Rs
where Rs is the absolute reflectance of the surface of the object 11.
Since the amount of light to be measured is the product of the reference amount of light and the absolute reflectance of the object, the absolute reflectance of the object Rs is EQU Rs=ls/lr
When the transmittance or reflectance is measured, the accuracy of measurement is increased by adopting a double-beam method. In the double-beam method, a light beam from a light-source is divided into two beams: one beam travels an optical path including the object, another travels an optical path including the light receptor. By monitoring the optical path including the light receptor, information such as the intensity fluctuations of the light source is measured, thereby correcting measured values of the object and increasing the accuracy of measurement.
The conventional apparatus for measuring the relative reflectance include the following apparatus. Referring to FIGS. 27(A) and 27(B), Mirrors M1 and M2 are inclined each other so as to form a shape like a gable roof. The reflecting surfaces of mirrors M1 and M2 are turned outward each other, resulting in the reflecting surfaces of the two mirrors being directed upward. A reference object 15 (as shown in FIG. 27(A)) or an object 11 (as shown in FIG. 27(B)) is placed above the mirrors M1 and M2. The light beam impinges on the Mirror M1 and is reflected by the same and travels to the reference object 15. And the light beam is reflected by the reference object 15 (or the object 11) and travels to the Mirror M2. After the light beam is reflected by the mirror M2, it travels to the light receptor (not shown). The light receptor measures lr, i.e., the amount of light, when the reference object 15 is placed above the mirrors M1 and M2, and measures ls, i.e., the amount of light, when the object 11 is placed above the mirrors M1 and M2. The relative reflectance is determined by Rs=ls/lr, as well as the previously-mentioned absolute reflectance.
In the case of performing ellipsometry using ellipsometer, since it is necessary to irradiate light to the object in an arbitrary angle and to receive the reflected light from the object, the following configuration is used as shown in FIG. 28.
The object 11 is placed on a goniostage 17 of the ellipsometer. A light source unit 19 is fixed. The light source unit 19 includes a light source 19a, a wave filter 19b, a light-collecting optical system 19c and a polarizer 19d. The light receptor 21 is fixed to a stage 23 that rotates about the same rotating axis as that of the goniostage 17, with rotation of the stage 23 being synchronized with rotation of the goniostage 17. The light receptor 21 includes an analyzer 21a, light-collecting system 21b and a light-receiving element 21c. The angle of incidence on a surface 11a of the object 11 is changed by rotating the goniostage 17. When the angle of incidence is .theta., the stage 23, on which the light receptor 21 is mounted, receives the reflected light from the object 11 after the stage 23 rotates 2.theta. degrees. Thus, it is possible to perform photometry by changing an angle of incidence.
To calculate the phase change of light from information measured using ellipsometer, it is necessary to use the specific method and calculation theory of ellipsometry. Although the method and calculation are described in reference 1 ("Optical Measurement Handbook" by Toshiharu Takou, Asakura book publishing), explanations of them are omitted because they are not directly related to the present invention.
However, the above-described devices for measuring optical properties have the following problems. One problem is that the absolute reflectance measuring apparatus as shown FIG. 26 cannot change the angle of incidence to measure the reflectance. To solve this problem, it is necessary, every time measurement is performed, to prepare the VN optical system having a different angle of incidence. However, even if such a VN optical system is prepared, the reflectance can be measured at a merely discrete angle of incidence. Thus, the conventional device cannot measure the reflectance at an arbitrary angle of incidence, and cannot change the angle of incidence continuously to measure the reflectance.
Another problem is that the relative reflectance measuring apparatus, as shown in FIG. 27, also cannot change the angle of incidence to measure the reflectance. To solve this problem, it is necessary to prepare an optical system shaped like a gable roof every time the angle of incidence is changed. However, even if the optical system shaped like the gable roof is prepared every time the angle of incidence is changed, the reflectance can be measured at a merely discrete angle of incidence. Thus, the conventional devices cannot measure the reflectance at an arbitrary angle of incidence, and cannot change an angle of incidence continuously to measure the reflectance.
Furthermore, as to both apparatus as shown in FIGS. 26 and 27, the VN optical system or the optical system shaped like the gable roof must be exchanged every time the angle of incidence is changed, and the exchanged optical system must be adjusted. This causes a problem that optical properties, such as optical path difference, aberration, focal length and quantity of light, could be changed, strictly speaking.
Furthermore, using an ellipsometer could cause instability of measurement because of mechanical causes, because it is necessary for a mechanical structure to move the light receptor while moving the object. Moving the light receptor to a different position could make it difficult to achieve high accuracy of measurement, because magnetic field distribution could affect adversely. Furthermore, because of above-mentioned mechanical structure, a larger space is necessary for installing the apparatus.
Furthermore, the essential disadvantage of the ellipsometer is that it cannot perform double beam measurement. Accordingly, it cannot compensate fluctuation of intensity of light of the light source, or fluctuation of intensity of polarized light.
The present invention is made in consideration of above-described problems. An object of the present invention is to provide a unit 30 for measuring optical property that can measure at least one of transmittance, reflectance and phase change in transmission or reflectance, more easily than before, without changing optical property of an optical system or moving a light receptor.